[Rcpp-devel] R Session Aborted when calling C++

[Gregory Jefferis]

Alexander,

I think your bounds checking code uses bitwise & rather than logical &&

     if((neighbor_idx(0) < nr) & (neighbor_idx(1) < nc) & (neighbor_idx(0) >= 0) & (neighbor_idx(1) >= 0)){
        GLCM(focal_val,neighbor_val) = GLCM(focal_val,neighbor_val)+1;
        GLCM(neighbor_val,focal_val) = GLCM(neighbor_val,focal_val)+1;
      }
 

Best,

Greg.

Sent from my iPhone

> On 29 Oct 2020, at 00:15, Dirk Eddelbuettel <edd at debian.org> wrote:
> 
> 
> Alexander,
> 
> A segmentation fault can take the session down as it is undefined behavior,
> yet is almost always the error of the programmers.
> 
> We have written the basic accessors for efficiency so they do not check. But
> there are checking accessor that throw with a proper message:
> 
>  R> Rcpp::cppFunction("void foo(IntegerVector x) { int n = x.size(); Rcpp::Rcout << x.at(n+1) << std::endl; }")
>  R> foo(1:3)
>  Error in foo(1:3) : Index out of bounds: [index=4; extent=3].
>  R>
> 
> No segfault, just a clean error message. RcppArmadillo has something similar.
> 
> By contrast if you don't check you can get garbage or bad behavior:
> 
>  R> Rcpp::cppFunction("void bar(IntegerVector x) { int n = x.size(); Rcpp::Rcout << x[n+1] << std::endl; }")
>  R> bar(1:3)
>  1358954573
>  R>
> 
> No crash but not exactly 'right' either.
> 
> The other thing you can do is to couple R with gdb to run under the debugger
> to get access to your indexing variables. There are some writeups in
> different places as it takes two steps---but may be worth it. Here are two
> quick hits from StackOverflow:
> 
> https://stackoverflow.com/questions/21226337/what-are-productive-ways-to-debug-rcpp-compiled-code-loaded-in-r-on-os-x-maveri
> https://stackoverflow.com/questions/11345537/debugging-line-by-line-of-rcpp-generated-dll-under-windows
> 
> Dirk
> 
> -- 
> https://dirk.eddelbuettel.com | @eddelbuettel | edd at debian.org
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